Liquid crystalline display

ABSTRACT

The optical transitions of extrinsically optically active insoluble materials become optically active and circularly dichroic when in contact with optically negative liquid crystalline materials thereby providing unusual and highly advantageous properties. The circularly dichroic optical properties induced in the extrinsically optically active insoluble materials can be utilized for optical display applications.

BACKGROUND OF THE INVENTION

This information relates to liquid crystalline materials and, morespecifically, to uses of compositions comprising optically negativeliquid crystalline materials and insoluble extrinsically opticallyinactive materials which become optically active when in contact withoptically negative liquid crystalline materials.

Liquid crystalline substances exhibit physical characteristics some ofwhich are typically associated with liquids and others which aretypically unique to solid crystals. The name "liquid crystals" hasbecome generic to substances exhibiting these dual properties. Liquidcrystals are known to appear in three different forms: the smectic,nematic, and cholesteric forms. These structural forms are sometimesreferred to as mesophases thereby indicating that they are states ofmatter intermediate between the liquid and crystalline states. The threemesophase forms of liquid crystals mentioned above are characterized bydifferent physical structures wherein the molecules of the compound arearranged in a manner which is unique to each of the three mesomorphicstructures. Each of these three structures is well known in the liquidcrystal art.

Some liquid crystalline substances possess optically negativecharacteristics. Birefringence, also referred to as double refraction,is an optical phenomenon characteristic of some solid crystals and mostliquid crystal substances. When a beam of unpolarized light strikes abirefringent substance it is split into two polarized components whosetransverse vibrations are at right angles to each other. The twocomponents are transmitted at different velocities through the substanceand emerge as beams of polarized light. By the term "liquid crystallinesubstances which have optically negative characteristics", as usedherein, is meant those for which the extraordinary index of refractionη_(E) is smaller than the ordinary index of refraction η_(o).Cholesteric liquid crystal substances exhibit this property. For adetailed description of this phenomenon, see Optical Crystallography,Wahlstrom, Fourth Edition, Wiley and Sons, Inc., New York.

The molecules in cholesteric liquid crystals are arranged in very thinlayers with the long axes of the molecules parallel to each other and tothe plane of the layers within each layer. Because of the asymmetry andsteric nature of the molecules, the direction of the long axes of themolecules in each layer is displaced slightly from the correspondingdirection in adjacent layers. This displacement is cumulative oversuccessive layers so that overall displacement traces out a helicalpath. A comprehensive description of the structure of cholesteric liquidcrystals is given in Molecular Structure and the Properties of LiquidCrystals, G. W. Gray, Academic Press, 1962.

Cholesteric liquid crystals have the property that when the propagationdirection of plane polarized or unpolarized light is along the helicalaxis thereof, i.e., when the light enters in a direction perpendicularto the long axes of the molecules, (neglecting absorptionconsiderations), this light is essentially unaffected in transmissionthrough thin films of such liquid crystals except for a wavelength bandcentered about some wavelength λ_(o) where λ_(o) = 2np with nrepresenting the index of refraction of the liquid crystal substance andp the pitch or repetition distance of the helical structure. Thebandwidth Δλ_(o) of this wavelength band centered about λ_(o) willtypically be of the order of about λ_(o) /14. For light of a wavelengthλ_(o), the cholesteric liquid crystal, under these conditions, exhibitsselective reflection of the light such that approximately 50% of thelight is reflected and approximately 50% is transmitted, assumingnegligible absorption which is usually the case, with both the reflectedand transmitted beams being approximately circularly polarized inopposite directions.

For light having wavelengths around λ_(o) but not at λ_(o), the sameeffect is present but not as pronounced. The transmitted light is notcircularly polarized but is instead elliptically polarized. Thecholesteric liquid crystals which exhibit this property of selectivereflection of light in a region centered around some wavelength λ_(o)are said to be in the Grandjean or "disturbed" texture. If λ_(o) is inthe visible region of the spectrum, the liquid crystalline film appearsto have the color corresponding to λ_(o) and if λ_(o) is outside thevisible spectral region, the film appears colorless.

Depending upon the intrinsic rotary sense of the helix, i.e., whether itis right-handed or left-handed, the light that is transmitted in theregion about λ_(o) is either right-hand circularly polarized light(RHCPL) or left-hand circularly polarized light (LHCPL). The transmittedlight is circularly polarized with the same sense of polarization asthat intrinsic to the helix. Thus, a cholesteric liquid crystal havingan intrinsic helical structure which is left-handed in sense willtransmit LHCPL and one having a helical structure which is right-handedin sense will transmit RHCPL.

Hereinafter, these cholesteric liquid crystal substances will beidentified in order to conform with popular convention, by the kind oflight which is reflected at λ_(o). When a film is said to beright-handed, it is meant that it reflects RHCPL, and when a film issaid to be left-handed, it is meant that it reflects LHCPL.

A right-handed cholesteric liquid crystal substance transmits LHCPLessentially completely at λ_(o) whereas the same substance reflectsalmost completely RHCPL. Conversely, a left-handed film is almosttransparent to RHCPL at λ_(o) and reflects LHCPL. Since plane polarizedor unpolarized light contain equal amounts of RHCPL and LHCPL, acholesteric liquid crystal film is approximately 50% transmitting atλ_(o) for these sources when the liquid crystal is in its Grandjeantexture.

A further unique optical property of optically negative liquid crystalfilm is that contrary to the normal situation when light is reflected,such as by mirror, where the sense of the circular polarization of thereflected light is reversed, this same phenomenon does not occur withlight reflected by these liquid crystal films. The sense of the circularpolarization of light reflected from these liquid crystal substances isnot reversed but rather remains the same as it was before it came intocontact with the liquid crystal substance. For example, if RHCPL havinga wavelength λ_(o) = 2np is directed at a right-hand film, it issubstantially completely reflected and, after reflection, remains RHCPL.If the same light were to be directed on a metallized mirror, inreflected light would be LHCPL.

Because of these optical properties, optically negative liquidcrystalline substances have been found to be highly advantageous for usein a number of applications. U.S. Pat. Nos. 3,669,525 and 3,679,290disclose the use of such liquid crystalline materials in optical filtersystems. U.S. Pat. No. 3,744,920 discloses the use of these materials ina detection system which can identify physical surface and/or electricalconductivity irregularities in a surface of interest.

Extremely large extrinsic circular dichroism has been observed withinthe electronic transitions of achiral (optically inactive) solutesdissolved in cholesteric mesophases as reported in recently issued U.S.Pat. No. 3,780,304 to F. D. Saeva et al. and in the following articlesby F. D. Saeva et al. appearing in the Journal of the American ChemicalSociety (JACS): "Cholesteric Liquid-Crystal-Induced Circular Dichroism(LCICD) of Achiral Solutes. A Novel Spectroscopic Technique", Vol. 94,JACS, page 5135 (1972); "Cholesteric Liquid-Crystal-Induced CircularDichroism (LCICD). V. Some Mechanistic Aspects", Vol. 95, JACS, page7656 (1973); "Cholesteric Liquid-Crystal-Induced Circular Dichroism(LCICD). VI. LCICD Behavior of Benzene and Some of its Mono- andDisubstituted Derivatives", Vol. 95, JACS, page 7660 (1973); and"Cholesteric Liquid-Crystal-Induced Circular Dichroism (LCICD). VII.LCID of Achiral Solutes in Lyotropic Cholesteric Mesophases", Vol. 95,JACS, page 7882 (1973).

Circular dichroism has not been previously reported as induced inextrinsically optically active insoluble materials and it hasheretobefore been thought by those working in the art as evidenced bythe above articles that two mechanisms were important to the existenceof Liquid Crystal Induced Circular Dichroism in dissolved materials: (1)helical organization of solute, and (2) the exposure of solute to ahelical organization of liquid crystal molecules. Shortly after theinvention of this Application, data was reported which indicated thatmechanism (1) was not required for the observation of extrinsic LCICDwithin solutes in the cholesteric mesophase. That is, the solutemolecules need not be ordered into helical organization by the mesophasein order to exhibit liquid crystal induced circular dichroism. The datais reported in "The Optical Activity of Achiral Molecules in aCholesteric Solvent", J.C.S. Chem. Comm., page 712, 1973.

It is known that the pitch of cholesteric liquid crystalline substancesis responsive to various foreign stimuli such as heat, pressure,electric fields, magnetic fields, etc. In some cases this characteristicis a highly desirable advantage, such as where the substance is used ina detection system to indicate the presence, or a change in the amountpresent, of any particular stimulus. However, according to some uses ofthese substances, the fact that their performance is affected by foreignstimuli is not an advantage and it would be desirable to have materialswhose performance in a particular mode would be essentially independentof the presence of the above mentioned stimuli.

In rapidly growing areas of technology such as liquid crystals newmethods, apparatus, compositions and articles of manufacture are oftendiscovered for the application of the new technology in a new mode. Thepresent invention relates to novel and advantageous uses of normallyextrinsically optically inactive insoluble materials in contact withoptically negative liquid crystalline materials.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide an opticalsystem having the above mentioned desirable features.

It is another object of the invention to provide an optical systememploying optically negative liquid crystalline compositions whichsystem is operative essentially independently of the presence of foreignstimuli.

It is still a further object of the invention to provide a novel opticaldisplay.

The above mentioned objects and advantages and others are realized inaccordance with the invention by employing optically negative liquidcrystalline substances in contact with extrinsically optically activeinsoluble materials whose electronic transitions, i.e., effect obtainedfrom the interaction of light energy with the electrons of themolecules, become circularly dichroic (i.e., show a large preferentialabsorption of either LHCPL or RHCPL) when in contact with an opticallynegative liquid crystalline material. It should be understood that bythe term "extrinsically optically active insoluble materials" we meanboth intrinsically optically active and intrinsically optically inactiveinsoluble materials which are optically active or inactive,respectively, out of contact (or when not in contact) with opticallynegative liquid crystalline substances. Both intrinsically opticallyactive insoluble materials and optically inactive insoluble materialsbecome extrinsically optically active when contacted with opticallynegative liquid crystalline material. This extrinsic induced behavioroverwhelms and dominates over any intrinsic activity since the specificrotations and molecular ellipticities in the former case aresubstantially larger than that observed for intrinsically opticallyactive insoluble materials.

"Insoluble" is used herein to mean that dissolution of the materialadded to or contacted by the optically negative liquid crystallinematerial can not be detected by conventional photometric techniques,such as circular dichroism and optical absorption.

It has been found that when such extrinsically optically activeinsoluble materials are placed in or otherwise contacted by cholestericmesophases, optical activity is induced in the optically inactivematerials and they exhibit circularly dichroic behavior within theirabsorption bands. The optical behavior induced in the normally opticallyinactive insoluble materials is due to absorption whereas the circularlydichroic behavior of optically negative liquid crystalline substances isdue to selective reflection of one type of circularly polarized light.The insoluble materials which acquire the induced optical activity, asopposed to the optically negative liquid crystalline substances, absorbboth RHCPL and LHCPL; however, they show a large preference for one typemore than for the other type.

Thus, such insoluble materials may be utilized in devices which can betuned to the absorption band of the insoluble materials rather than thereflection band of the optically negative liquid crystalline substance;or use may be made of both absorption bands of such insoluble materialsand reflection bands of optically negative liquid crystalline materials.

The invention will be more fully understood from the following detaileddescription of various preferred embodiments thereof particularly whenread in conjunction with the accompanying drawings wherein:

FIG. 1 shows the circular dichroism and absorption spectra of CalciumBonadur Red particles suspended in the cholesteric mesophase ofcholesteryl oleyl carbonate.

FIG. 2 shows the circular dichroism and absorption spectra of solubleand insolublilized anthracene-9-carboxylic acid in the cholestericmesophase of cholesteryl chloride (60 wt. %) - cholesteryl nonanoate (40wt. %).

FIG. 3 is a schematic isometric partially exploded view of oneembodiment of the invention.

FIG. 4 is a schematic isometric view, partially exploded, of anotherembodiment of the invention.

Referring now to FIG. 1, there is seen the circular dichroism andabsorption spectrum of a thin film (about 10 microns thick) of insolubleCalcium Bonadur Red pigment particles suspended in the cholestericmesophase of cholesteryl oleyl carbonate. The pigment particles areinsoluble in the cholesteryl oleyl carbonate and are much larger in sizethan the molecules of cholesteryl oleyl carbonate. Surprisingly, it wasfound that the insoluble particles exhibited liquid crystal inducedcircular dichroism not withstanding the fact that their large sizerelative to the molecules of the cholesteric mesophase prevented theirbeing ordered into helical organization by the helical array ofmolecules of the cholesteric mesophase.

Bands of negative sign (E_(R) > E_(L)) appear at about 520 and about 570nm wavelengths of light in the circular dichroism spectrum. A major peakof absorption appears at about 580 nm of light and a shoulder at about520 nm of light in the absorption spectrum. These wavelengths are withinthe visible region. As seen from the procedure of Example I, below, theabsorption and circular dichroism exhibited within the visible region isattributable solely to circular dichroism induced in the particles. Itwas further found, as seen from comparing particles sizes in Examples 1and 2, below, that the phenomenon of circular dichroism induced in theparticles is a surface phenomenon. The ratio of circular dichroism tooptical density increases in intensity with increase in surface areaprovided by the particles. That is, for the same weight amount ofinsoluble particles suspended in the cholesteric mesophase, a greaterratio is exhibited by smaller sized particles than by larger sizedparticles. The sign of extrinsic circular dichroism in the particles isindependent of the position of the cholesteric pitch band λ_(o).

FIG. 2 presents for comparison the absorption and circular dichroismspectra of soluble and insolubilized anthracene-9-carboxylic acid. It isnoted that while the soluble anthracene-9-carboxylic acid in thecholesteric mesophase (60 wt. % cholesteryl chloride - 40 wt. %cholesteryl nonanoate) exhibits a change in sign in circular dichroismwhich is dependent upon the position of the cholesteric pitch bandλ_(o), the insolubilized anthracene-9-carboxylic acid remains positivein sign in circular dichroism (E_(L) > E_(R)) notwithstanding change inposition of the cholesteric pitch band λ_(o). Previously, it wasobserved with solutes in cholesteristics that the sign of circulardichroism induced in the solute was dependent upon the handedness of thecholesteric pitch band λ_(o).

Of course, it will be recognized that the particular insoluble materialsof FIGS. 1 and 2 are typical of the insoluble optically inactivematerials of the invention and are used to illustrate what effect isobtained; similar results can be obtained with any of the insolubleoptically inactive materials encompassed by the invention.

Experimental results with insoluble materials indicate that theintensity of the induced circularly dichroic absorption band varies withvariation in pitch of the cholesteric mesophase, as well as with thechirality of the cholesteric helix. The sign of the extrinsic circulardichroism changes with chirality of the cholesteric helix. However, thesign of the extrinsic dichroism is independent of the wavelengthlocation of the optically negative liquid crystalline pitch band λ_(o)relative to the wavelength location of the absorption band of theinsoluble material.

An important advantage derived from exploiting the induced circulardichroic optical activity of the absorption band of the insolublematerials contacted with the optically negative liquid crystallinematerial is that the absorption band will always remain substantially inthe same position and will not be shifted to any significant extent bythe presence of foreign stimuli. The magnitude of the optically activeeffect will typically change when a foreign stimulus acts upon thecomposition but the position of the band will not. This behavior isopposite to that of the pitch band of the optically negative liquidcrystalline composition when acted upon by a foreign stimulus since, asis appreciated by those skilled in the art, the location of the pitchband changes but the amplitude thereof is always substantially the same.For example, when a stimulus acts upon the optically negative liquidcrystalline environment, the pitch may become larger causing λ_(o) tobecome larger (since λ_(o) = 2np).

Thus, it can be seen that the addition of extrinsically optically activeinsoluble materials whose absorption bands become highly opticallyactive when in contact with an optically negative liquid crystallineenvironment permits a novel and highly advantageous means for tailoringthe properties of optically negative liquid crystal systems to achievenovel and extremely useful results. The above mentioned additives can beused to provide a circularly dichroic absorption band for thecomposition.

The additives which can be placed in contact with optically negativeliquid crystalline substances according to the invention should beinsoluble (as previously defined) in such a liquid crystallineenvironment and should have optical transitions which become circularlydichroic in some region of the electromagnetic spectrum. Any suitableextrinsically optically active insoluble material can be used accordingto the invention. Typical suitable extrinsically optically activeinsoluble materials include, among others, organic and inorganicpigments, aromatic insoluble compounds such as insolubilized benzene,naphthalene, anthracene and the like; insoluble azo compounds such asinsolubilized arylazonaphtols, azobenzenes, etc.; insoluble nitrocompounds such as insolubilized nitrobenzene, nitroarylazonaphthols andthe like; insoluble nitroso compounds such as insolubilizednitrosonaphthalene and the like; insoluble compounds such asinsolubilized benzylidene aniline, etc.; insoluble carbonyl compoundssuch as insolubilized acetone, acetophenone, benzophenone and the like;insoluble thiocarbonyls such as insolubilized thioacetophenones,thioacetone, thiobenzophenone and the like; insoluble alkenes such asinsolubilized butadiene, cyclohexane, etc.; insoluble heterocyclics suchas insolubilized furans, aziridines, pyridines and the like, insolublealkanes such as insolubilized hexane, dodecane and the like; metalliccomplexes; dyes such as polymethin, sulfur, indigo and anthraquinonedyes; and mixtures thereof.

Typical methods of insolubilizing include adsorbing on suitable surfacesand converting to ionic derivatives.

Generally speaking, it is preferred to use extrinsically opticallyactive additive materials that absorb in the visible region of thespectrum such as, for example, inorganic and organic pigments in thenovel compositions of the invention since the colored additives willprovide preferred results when the compositions are utilized in variousmodes of application as will be discussed in detail hereinafter. Forexample, in a preferred embodiment of the invention where thecompositions of the invention are employed in an imaging mode the use ofcolored additive materials will permit readout in the visible region ofthe spectrum of an image where the optical input is not in the visiblespectral region of the electromagnetic spectrum.

Of course, it should be recognized that the above classes of materialsare intended to be illustrative only of the insoluble additives whichwill provide the previously described induced behavior.

The amount of insoluble material which can be incorporated into anoptically negative liquid crystalline can vary over an extremely widerange. The amount added in any particular instance is dependentprimarily upon the intended use of the particular composition. Forexample, where it is intended to exploit the induced optical activity ofthe absorption band of the additive as little as up to about 10% byweight of optically inactive material can provide the induced opticalactivity. Of course, the upper limit of the amount of additive which canbe incorporated into any particular optically negative liquid crystalcomposition, and which can go as high as about 90% by weight, iscontrolled by the requirement that the total environment must retain itsoptically negative liquid crystalline character after the addition ofthe optically inactive material.

Any suitable cholesteric liquid crystal substance, mixtures thereof orcompositions having liquid crystalline characteristics may be employedin the invention. Typical suitable cholesteric liquid crystals includederivatives from reactions of cholesterol and inorganic acids, forexample: cholesteryl chloride, cholesteryl bromide, cholesteryl iodide,cholesteryl fluoride, cholesteryl nitrate; esters derived from reactionsof cholesteryl and carboxylic acids, for example, cholesteryl crotonate;cholesteryl nonanoate, cholesteryl hexanoate; cholesteryl formate;cholesteryl docosonoate; cholesteryl proprionate; cholesteryl acetate;cholesteryl valerate; cholesteryl vacconate; cholesteryl linolate;cholesteryl linolenate; cholesteryl oleate; cholesteryl erucate;cholesteryl butyrate; cholesteryl caproate; cholesteryl laurate;cholesteryl myristate; cholesteryl clupanodonate; ethers of cholesterolsuch as cholesteryl decyl ether; cholesteryl lauryl ether; cholesteryloleyl ether; cholesteryl dodecyl ether; carbamates and carbonates ofcholesterol such as cholesteryl decyl carbonate; cholesteryl oleylcarbonate; cholesteryl methyl carbonate; cholesteryl ethyl carbonate;cholesteryl butyl carbonate; cholesteryl docosonyl carbonate;cholesteryl cetyl carbonate; cholesteryl-p-nonylphenyl carbonate;cholesteryl-2-(2-ethoxyethoxy) ethyl carbonate;cholesteryl-2-(2-butoxyethoxy) ethyl carbonate;cholesteryl-1-2-(2-methoxyethoxy) ethyl carbonate; cholesteryl geranylcarbonate; cholesteryl heptyl carbamates; and alkyl amides and aliphaticsecondary amines derived from 3β-aminoΔ5-cholestene and mixturesthereof; peptides such as poly-γ-benzyl-L-glutamate; derivatives of betasitosterol such as sitosteryl chloride; and amyl ester of cyanobenzilidene amino cinnamate. The alkyl groups in said compounds aretypically saturated or unsaturated fatty acids, or alcohols, having lessthan about 25 carbon atoms, and unsaturated chains of less than about 5double-bonded olefinic groups. Aryl groups in the above compoundstypically comprise substituted benzene ring compounds. Any of the abovecompounds and mixtures thereof may be suitable for cholesteric liquidcrystalline materials in the advantageous system of the presentinvention.

Mixtures of liquid crystals can be prepared in organic solvents such aschloroform, petroleum ether and others, which are typically evaporatedfrom the mixture leaving the liquid crystal composition. Alternatively,the individual components of the liquid crystalline mixture can becombined directly by heating the mixed components above the isotropictransition temperature.

The above lists of typical suitable optically negative liquidcrystalline substances are intended to encompass mixtures of the above.These lists are intended to be representative only and are not to beconstrued as being exhaustive or limiting the invention to the specificmaterials mentioned. Although any liquid crystalline composition havingcholesteric liquid crystalline characteristics is suitable for use inthe present invention, it should be recognized that various differentcholesteric liquid crystal substances or mixtures thereof orcombinations of cholesteric liquid crystal substances with othersubstances such as those mentioned above will only possess the necessaryproperties which make them suitable for use according to the inventionat some specific temperature range which may be at room temperature orsubstantially below or above room temperature. However, all of thevarious substances, mixtures or combinations thereof will functionaccording to the method at some temperature. Typically, the materials ofthe invention will be used at or near room temperature. Thus, it ispreferred to employ liquid crystal substances which have a liquidcrystal state at or near room temperature. Generally speaking, theliquid crystal substance will preferably be in the liquid crystal stateat the desired operational temperature.

Typical suitable thicknesses of films or layers of optically negativeliquid crystalline material are from about 0.5 to about 50 microns,although any thickness which will provide the desired effect can beused.

The invention will now be described further in detail by way ofexamples, it being understood that these are intended to be illustrativeonly and the invention is not limited to the conditions, materials,procedures, etc., recited therein. All parts and percentages listed areby weight unless otherwise specified.

EXAMPLE I

The calcium salt of3'-ethyl-4'-chloro-6'-sulfonylphenylazo-2-hydroxy-3-naphthoic acid(Calcium Bonadur Red) is finely ground between two planes of groundglass to an average particle size of about 10 microns. About 0.1 gm ofthe particles are uniformly dispersed in about 11 gm of the cholestericmesophase of cholesteryl oleyl carbonate. The dispersion is placedbetween quartz plates about 1 × 1 × 1/8 inch in dimension. The preparedsample is analyzed with a Carey 15 Spectrophotometer for absorptionspectrum. The prepared sample is analyzed with a Carey 61Spectropolarimeter for circular dichroism. FIG. 1 graphicallyillustrates the resulting circular dichroism spectrum.

The absorption spectrum shows a major peak at about 580 nm and ashoulder at about 520 nm. The circular dichroism spectrum shows bands ofnegative sign (E_(R) > E_(L)) at about 520 nm. The negative sign doesnot change with polarization of the optical transition.

The dispersion was then centrifuged and the dispersed particles therebyseparated from the cholesteryl oleyl carbonate. The cholesteryl oleylcarbonate is then re-examined spectrophotometrically and shows noabsorption or circular dichroism in the visible region of the spectrum.This eliminates the possibility of particles having dissolved in thecarbonate and indicates that circular dichroism arises from a surfaceeffect in the particles.

Example I demonstrates that circular dichroism can be induced ininsoluble materials by dispersing same in an optically negative liquidcrystalline material.

EXAMPLE II

Example I is repeated except that the particles are ground to an averageparticle size of about 2 microns. The ratio of circular dichroism tooptical density is determined to be larger with these about 2 micronparticles than with the about 10 micron particles of Example I. Thisbuttresses the indication of Example I that the induced circulardichroism in insoluble materials is a surface phenomenon because thesmaller particles present a larger surface area in contact with theoptically negative liquid crystalline substance.

Except for the difference in the ratio, the same results are obtained inExample II as are obtained in Example I.

EXAMPLE III

Two samples are prepared as follows: each sample contains thecholesteric mesophase of 60% cholesteryl chloride - 40% cholesterylnonanoate. In sample I, anthracene-9-carboxylic acid is dissolved in themesophase. In sample II, particles of insoluble calciumanthracene-9-carboxylic acid are dispersed in the mesophase. Theabsorption and circular dichroism spectrums of samples I and II aredetermined in the manner of Example I. FIG. 2 graphically illustratesthe resulting spectra.

The circular dichroism induced in the dissolved acid in sample I changessign at about 355 nm indicating a change in polarization of theelectronic transitions. The dispersed insoluble particles of calciumanthracene-9-carboxylic acid in sample II exhibit induced circulardichroism which does not change sign with polarization of the electronictransition.

The circular dichroism of insoluble particles in sample II exhibitsbands which are slightly blue shifted from the absorption bands for theparticles. The relative intensities of the circular dichroism bands ofthe particles in sample II are more similar to the relative intensitiesof the absorption bands of the dissolved acid in sample I than to thoseof the absorption bands of the insoluble particles in sample II. Thesign of the circular dichroism induced in the insoluble particles ofsample II is positive (E_(L) > E_(R)) and independent of whether λ_(o)for the cholesteric mesophase is at larger or smaller wavelengths thanthe absorption band of the particles.

The difference in depending upon position of λ_(o) between the dissolvedacid in sample I and undissolved particles in sample II indicates adifference in mechanism between induced circular dichroism in solutesand induced circular dichroism in insoluble materials in intimatecontact with optically negative liquid crystalline materials.

EXAMPLE IV

Vanadyl phthalocyanine (VOPC) is heated in a vacuum to sublime a film ofVOPC upon a quartz disc about 1 × 1 × 1/8 inch. The VOPC film isovercoated with a layer of the cholesteric mesophase of cholesteryloleyl carbonate (COC) which, in turn, is contacted with a glass plate toproduce a glass-VOPC-COC-quartz disc sandwich.

The absorption and circular dichroism spectrum of the COC-VOPCcombination was examined and circular dichroism was observed in theregion of the visible electronic transitions of VOPC. This observationdemonstrates that circular dichroism induced in insoluble materials inintimate contact with optically negative liquid crystalline materials isdue to specific interaction between the two materials.

EXAMPLE V

Example IV is repeated except that the VOPC is replaced with copperphthalocyanine (CuPC). Circular dichroism is observed in the visibleelectronic transitions of CuPC.

EXAMPLE VI

Example I is followed except that Bonadur Red is replaced by copperphthalocyanine (CuPC).

The absorption spectrum shows major peaks at about 600 nm and about 690nm. The circular dichroism spectrum shows bands of positive sign(E_(R) > E_(L)) at about 595 nm and about 680 nm.

The CuPC particles are centrifuged out and the liquid crystallinematerial re-examined spectrophotometrically; no absorption or circulardichroism in the visible region is exhibited. This eliminates thepossibility that circular dichroism was exhibited by dissolved moleculesrather than by insoluble particles.

EXAMPLE VII

Particles of calcium anthracene-9-carboxylic acid are dispersed in asupercooled cholesteric mesophase of 60% cholesteryl chloride - 40%cholesteryl nonanoate. The dispersion is sandwiched at a thickness ofabout 7 microns between two tin-oxide coated quartz discs 1 × 1 × 1/8inch. A D.C. voltage is applied to the oxide coatings to apply anelectric field across the dispersion while the dispersion is beingexamined for circular dichroism induced in the particles. Uponapplication of the field, a change in both sign and magnitude of theinduced circular dichroism is observed within the absorption bands ofthe dispersed particles.

When about 30 volts D.C. are applied to the oxide coatings, the sign ofthe induced circular dichroism goes from positive to negative and theintensity is reduced by about one-third of its original value (i.e.,value prior to field application).

The circular dichroism intensity is substantially completely eliminatedwhen the applied voltage is about 400 volts D.C..

Example VII demonstrates that the application of electric fields acrosscholesteric mesophases in contact with an insoluble material, altersboth the sign and magnitude of the induced (extrinsic) circulardichroism.

EXAMPLE VIII

Example VII is followed except that one of the tin oxide coated quartzdiscs is coated with tin oxide in imagewise configuration. A D.C.voltage of about 400 volts is applied to the tin oxide coatings causinga transition of the 60/40 CC-CN from optically negative characteristicsto optically positive characteristics in portions thereof sandwichedbetween the tin oxide coatings. The anthracene-9-carboxylic acidparticles suspended in the portions of the 60/40 CC-CN wherein suchtransition occurs exhibit no induced circular dichroism; whereas, theparticles in portions of the 60/40 CC-CN which do not undergo suchtransitions exhibit induced circular dichroism. Thus an image of inducedcircular dichroism complementary to the imagewise configured tin oxidecoating is formed.

EXAMPLE IX

Example VIII is followed except that the imagewise configured tin oxidecoating on the one quartz disc is replaced by a tin oxide coating on aquartz disc which corresponds to background areas only. That is, thedeposition of tin oxide is in a pattern complementary to that in ExampleVIII. Transition of the 60/40 CC-CN from optically negativecharacteristics to optically positive characteristics occurs in theportions thereof sandwiched between tin oxide coatings. Induced circulardichroism is not exhibited by the particles suspended in suchtransitioned portions but is exhibited by particles suspended innon-transitioned portions of 60/40 CC-CN (i.e., those not sandwichedbetween tin oxide coatings). Thus, an image of induced circulardichroism is formed which corresponds to the non tin-oxide coated areasof the quartz disc bearing the patterned deposition of tin oxide.

For a greater appreciation of the significance of comparative ExampleIII above, reference is made to the aforementioned article by F. Saevaappearing in 95 Journal of American Chemical Society, page 7656 (1973);especially Table I and accompanying text on page 7657 wherein thedependency of liquid crystal induced circular dichroism in solutes uponhelix sense of the cholesteric mesophase upon direction of electronictransition moments of solute and upon cholesteric mesophase pitch bandλ_(o) position relative to wavelength of absorption of solute, isdiscussed in detail. That article is hereby incorporated by reference.

It will be appreciated, of course, that all embodiments of the inventionherein described can be used with or without application of anelectrical or magnetic field, and that such usage may include selectiveapplication of an electrical or magnetic field with regard to time ofapplication, duration of application, and area of application. It isnoted that the application of electrical or magnetic field strength ator above that causing transition of the liquid crystalline material fromoptically negative characteristics to optically positive characteristicsdestroys the circular dichroism induced in extrinsically opticallyactive materials in contact with the liquid crystalline materials

FIG. 3 schematically depicts a partially exploded view of one embodimentof the invention. A film 11 of optically negative liquid crystallinematerial having insoluble extrinsically optically active particles 30suspended therein is retained by protective casing 12. Plate 13 havingan electrode 14 on one side thereof is placed adjacent casing 12 so thatelectrode 14 is in contact with film 11. Plate 15 having a matrix ofdiscrete electrodes 16 on one side thereof is placed adjacent casing 11so that the electrode 16 is in contact with film 11. A switch mechanismS selectively electrically connects one or more of discrete electrodes16 to voltage source VS.

The optically negative liquid crystalline material of film 11 and theparticles 30 can comprise any of the aforementioned liquid crystallinematerials and extrinsically optically active materials, respectively.

Protective casing 12 and plates 13 and 15 may comprise any suitablematerial, flexible or rigid, which is optically isotropic andtransparent to the incident light radiation and which is non-reactivewith film 11. Typical suitable materials include glass, fused silica andany other materials having the required characteristics, such as quartz.

Electrodes 14 and 16 can comprise any suitable conductive material whichis non-reactive with film 11. For example, typical suitable conductivematerials include chrome and tin oxide.

FIG. 4 schematically depicts a partially exploded view of anotherembodiment of the invention wherein like numerals refer to likecomponents found in FIG. 3. However, FIG. 4 depicts two variations fromFIG. 3. The first is the film 31 of insoluble extrinsically opticallyactive materials. The second variation is the configuration of electrode16' on plate 15. Here, electrode 16' is in the range complementary tothe X pattern depicted. That is, plate 15 is not covered by electrode16' in surface areas bounded by the lines forming the individual legs ofthe X.

Film 31 may comprise any of the aforementioned insoluble extrinsicallyoptically active materials and may be formed by any method suitable tothe insoluble materials employed. For example, pigments such as copperphthalocyanine and vanadyl phthalocyanine are conveniently coated uponsubstrates by sublimation by being heated in an evacuated chamber. Othertypical suitable coating methods include roll coating, reverse rollcoating and coating with rods such as wire wound Mayer rods.

It is understood, of course, that FIGS. 3 and 4 are illustrative onlyand that particles 30 suspended in film 11 of FIG. 3 may be used withthe complementary imagewise configured electrode 16' of FIG. 4; that thefilm 31 of insoluble material of FIG. 4 may be used with the electrodes16 of FIG. 3; that other suitable imaging cells found in the art ofimaging liquid crystalline materials by application of an electricalfield may be employed provided a field strength sufficient to causetransition of the liquid crystalline material from optically negativecharacteristics to optically positive characteristics are employed.

As indicated by Examples VIII and IX, the circular dichroism induced inthe insoluble extrinsically optically active materials by the opticallynegative liquid crystalline material with which it is in contact isextinguished in portions of the liquid crystalline material transformedto optically positive characteristics. Thus, any suitable technique forso transforming may be employed. For example, U.S. Pat. No. 3,652,148herein incorporated by reference, provides a system for transformingoptically negative liquid crystalline materials to an optically positiveliquid crystalline mesophase by an applied electric field. This systemmay advantageously be employed in the practice of the instant invention.

Further, the liquid crystalline material and/or extrinsically opticallyactive insoluble material can be shaped in imagewise configuration inFIGS. 3 and 4 and placed between uniformly coated electrodes. With thefield off, the induced circular dichroism is in imagewise configurationcorresponding to the shaped liquid crystalline material in whichextrinsically optically active insoluble material is dispersed; and whenthe insoluble material is a layer shaped in image configuration, theinduced circular dichroism is in imagewise configuration correspondingto the shaped layer of insoluble material. In this latter case, theliquid crystalline material need not be in imagewise configuration.

While the invention has been described in detail with respect to variouspreferred embodiments thereof, it is not intended to be limited theretobut rather it will be appreciated by those skilled in the art thatmodifications and variations are possible which are within the spirit ofthe invention and the scope of the claims.

For example, those skilled in the art will appreciate that the displaymay be conveniently viewed by employing a quarter waveplate and linearpolarizer in accordance with the publication "Polarized Light" publishedby Polaroid Corporation, and hereby expressly incorporated by reference,particularly FIG. 7 and the accompanying description, No. FT3374A dated2/67.

What is claimed is:
 1. A method for providing a display, comprising:a.providing an imaging member comprising a layer of an optically negativeliquid crystalline material arranged between a pair of electrodes, atleast one of which is substantially transparent, said liquid crystallinematerial being in contact with at least one insoluble material, saidinsoluble material when in contact with said optically negative liquidcrystalline material becoming extrinsically optically active and havingan absorption band within which circular dichroism is induced, saidliquid crystalline material being at a temperature in itsnegative-positive optical transition range; b. applying an imagewiseconfigured electrical field across said layer within the opticallynegative-positive transition electrical field strength range of saidliquid crystalline material thereby extinguishing said induced circulardichroism in imagewise configuration and leaving induced circulardichroism in a pattern complementary to said imagewise configuration;and c. viewing said induced circular dichroism in said complementarypattern through a circular polarizer.
 2. The method as defined in claim1 wherein said circular polarizer comprises a linear polarizer andquarter waveplate.
 3. The method as defined in claim 1 wherein saidinsoluble material comprises a member selected from the insolubilizedgroup consisting of aromatic compounds, azo compounds, nitro compounds,nitroso compounds, anil compounds, carbonyl compounds, thiocarbonylcompounds, alkenes, heterocyclic compounds, alkanes and mixturesthereof.
 4. The method as defined in claim 1 wherein at least one ofsaid electrodes is shaped in said imagewise configuration.
 5. The methodas defined in claim 1 wherein said insoluble material is dispersedwithin said liquid crystalline material.
 6. The method as defined inclaim 5 wherein the insoluble material content of said liquid crystallayer comprises up to about 90% by weight.
 7. The method as defined inclaim 1 wherein said insoluble material is in a layer configuration. 8.A method for providing a display, comprising:a. providing an imagingmember comprising a layer of an optically negative liquid crystallinematerial shaped in image configuration between a pair of electrodes atleast one of which is substantially transparent, said liquid crystallinematerial having dispersed therein at least one insoluble material; saidinsoluble material becoming extrinsically, optically active and, havinga light absorption band within which circular dichroism is induced, bybeing in contact with said optically negative liquid crystallinematerial; said liquid crystalline material being at a temperature in thenegative-positive transition range of said liquid crystalline material;and b. while viewing said imaging member through a circular polarizer,applying an electrical field across said layer within the opticallynegative-positive transition electrical field strength range of saidliquid crystalline material.
 9. The method as defined in claim 8 whereinsaid circular polarizer comprises a linear polarizer and a quarterwaveplate.
 10. A method for providing a display, comprising:a. providingan imaging member comprising a layer of optically negative liquidcrystalline material in contact with a layer of insoluble materialshaped in image configuration, said layers between a pair of electrodesat least one of which is substantially transparent, said liquidcrystalline material being at a temperature in the negative-positiveoptical transition range of said liquid crystalline material; saidinsoluble material becoming extrinsically optically active and having alight absorption band within which circular dichroism is induced bycontact with said optically negative liquid crystalline material; and b.while viewing said imaging member through a circular polarizer, applyingan electrical field across said layers within the opticallynegative-positive transition range of said liquid crystalline material.11. The method as defined in claim 10 wherein said circular polarizercomprises a linear polarizer and a quarter waveplate.